In the early days of personal computing CPU bugs were so rare as to be newsworthy. The infamous Pentium FDIV bug is remembered by many, and even earlier CPUs had their own issues (the 6502 comes to mind). Nowadays they've become so common that I encounter them routinely while triaging crash reports sent from Firefox users. Given the nature of CPUs you might wonder how these bugs arise, how they manifest and what can and can't be done about them. 🧵 1/31

All in all modern CPUs are beasts of tremendous complexity and bugs have become inevitable. I wish the industry would be spending more resources addressing them, improving design and testing before CPUs ship to users, but alas most of the tech sector seems more keen on playing with unreliable statistical toys rather than ensuring that the hardware users pay good money for works correctly. 31/31

All in all modern CPUs are beasts of tremendous complexity and bugs have become inevitable. I wish the industry would be spending more resources addressing them, improving design and testing before CPUs ship to users, but alas most of the tech sector seems more keen on playing with unreliable statistical toys rather than ensuring that the hardware users pay good money for works correctly. 31/31

In the early days of personal computing CPU bugs were so rare as to be newsworthy. The infamous Pentium FDIV bug is remembered by many, and even earlier CPUs had their own issues (the 6502 comes to mind). Nowadays they've become so common that I encounter them routinely while triaging crash reports sent from Firefox users. Given the nature of CPUs you might wonder how these bugs arise, how they manifest and what can and can't be done about them. 🧵 1/31

Bonus end-of-thread post: when you encounter these bugs try to cut the hardware designers some slack. They work on increasingly complex stuff, with increasingly pressing deadlines and under upper management who rarely understands what they're doing. Put the blame for these bugs where it's due: on executives that haven't allocated enough time, people and resources to make a quality product.

The speed at which signals propagate in circuits is proportional to how much voltage is being applied. In older CPUs this voltage was fixed, but in modern ones it changes thousands of times per second to save power. Providing just as little voltage needed for a certain clock frequency can dramatically reduce power consumption, but providing too little voltage may cause a signal to arrive late, or the wrong signal to reach the pipeline register, causing in turn a cascade of failures. 24/31

@gabrielesvelto nitpick: the propagation velocity of a *signal* in a circuit is not affected by the voltage magnitude; that is a function of the (innate) dielectric constant of the material.

however, a higher core voltage does mean that a rising edge tends to reach the gate threshold voltage of a transistor more quickly, which reduces the time it takes for each asynchronous logic element's output to reach a well-defined state after a change in input, thus propagating logic *state* more quickly.

Bonus end-of-thread post: when you encounter these bugs try to cut the hardware designers some slack. They work on increasingly complex stuff, with increasingly pressing deadlines and under upper management who rarely understands what they're doing. Put the blame for these bugs where it's due: on executives that haven't allocated enough time, people and resources to make a quality product.

In the early days of personal computing CPU bugs were so rare as to be newsworthy. The infamous Pentium FDIV bug is remembered by many, and even earlier CPUs had their own issues (the 6502 comes to mind). Nowadays they've become so common that I encounter them routinely while triaging crash reports sent from Firefox users. Given the nature of CPUs you might wonder how these bugs arise, how they manifest and what can and can't be done about them. 🧵 1/31

In the early days of personal computing CPU bugs were so rare as to be newsworthy. The infamous Pentium FDIV bug is remembered by many, and even earlier CPUs had their own issues (the 6502 comes to mind). Nowadays they've become so common that I encounter them routinely while triaging crash reports sent from Firefox users. Given the nature of CPUs you might wonder how these bugs arise, how they manifest and what can and can't be done about them. 🧵 1/31

@gabrielesvelto (what you said is absolutely correct regarding "signals" in the HDL sense of the word, it just gets a bit muddled when we're simultaneously talking about the analogue behaviours of the actual electrical signals, hence the clarification ^^)

@gabrielesvelto Nice thread!

You seem to imply that bugs have become considerably more frequent, largely due to the increased complexity. Right?

To me it's not obvious that the larger number of known issues isn't to a large degree due to much better visibility (we didn't have anywhere close to today's automatic crash collection systems in the past) and due to the vastly increased number of CPUs... Do you have any gut feeling about that?

@AndresFreundTec I've been in charge of Firefox stability for ten years now and some of my early work to detect hardware issues dates back then. In pre-2020 years we could get a 2-3 bugs per year, usually across different CPUs. Now we get dozens, it's really on another level.

In the early days of personal computing CPU bugs were so rare as to be newsworthy. The infamous Pentium FDIV bug is remembered by many, and even earlier CPUs had their own issues (the 6502 comes to mind). Nowadays they've become so common that I encounter them routinely while triaging crash reports sent from Firefox users. Given the nature of CPUs you might wonder how these bugs arise, how they manifest and what can and can't be done about them. 🧵 1/31

@gabrielesvelto
There’s also meta-stability. If a value is snapshotted half way through it changing, it may occasionally result in the output not being one or zero, but some ‘half’ value. Depending on the circuits using that result, it may be interpreted as either 1 or 0 — and maybe different parts of the circuit will use different interpretations. Such intermediate states are only meta-stable, and will flip to a firm 1 or 0 at some indeterminate time later, possibly propagating the problem.

@gabrielesvelto fantastic thread thank you

@gabrielesvelto Fascinating thread, especially the degradation over time inherit to modern processors. That came up recently in an interesting viral video on a world where we forget how to make new CPUs.

Bit of an aside, but I assume this affects other architectures? The thread mentioned Intel and AMD, but I assume Arm and Risc-V are similarly prone to these sorts of problems?

However not all bugs can be fixed this way. Bugs within logic that sits on a critical path can rarely be fixed. Additionally some microcode fixes can only be made to work if the microcode is loaded at boot time, right when the CPU is initialized. If the updated microcode is loaded by the operating system it might be too late to reconfigure the core's operation, you'll need an updated UEFI firmware for some fix to work. 20/31

I can't be sure that this is exactly what's happening on Raptor Lake CPUs, it's just a theory. But a modern CPU core has millions upon millions of these types of circuits, and a timing issue in any of them can lead to these kinds of problems. And that's without saying that voltage delivery across a core is an exquisitely analog problem, with voltage fluctuations that might be caused by all sorts of events: instructions being executed, temperature, etc... 27/31

@gabrielesvelto Thank you for this detailed and specific explanation. Chris Hobbs discusses the relative unreliability of popular modern CPUs in "Embedded Systems Development for Safety-Critical Systems" but not to this depth.

I don't do embedded work but I do safety-related software QA. Our process has three types of test - acceptance tests which determine fitness-for-use, installation tests to ensure the system is in proper working order, and in-service tests which are sort of a mystery. There's no real guidance on what an in-service test is or how it differs from an installation test. Those are typically run when the operating system is updated or there are similar changes to support software. Given the issue of CPU degradation, I wonder if it makes sense to periodically run in-service tests or somehow detect CPU degradation (that's probably something that should be owned by the infrastructure people vs the application people).

I've mainly thought of CPU failures as design or manufacturing defects, not in terms of "wear" so this has me questioning the assumptions our testing is based on.